In recent years, an optical transmission system has been known in which a client signal output from a client apparatus is transmitted in the form of frames. A specific example of such a transmission method is an optical transport network (OTN) method that allows it to transparently transmit a client signal.
In a transmission apparatus using the OTN method, a client signal is mapped into a frame called an optical-channel data unit (ODU) frame, and ODU frames are multiplexed into a payload area of a frame called an optical-channel transport unit (OTU) frame. The transmission apparatus using the OTN method transmits the OTU frame including the ODU frames multiplexed in the payload area to a transmission apparatus on a receiving side. Hereinafter, an OTU frame including ODU frames multiplexed in a payload area thereof is referred to a multiplexed frame.
In an optical transmission system including a transmission apparatus using the OTN method, an increase in degradation of a multiplexed frame on a transmission line may occur with increasing transmission rate which is a tendency in recent years. In view of the above, various error correction algorithms have been investigated to correct an error of a degraded multiplexed frame. An example of such error correction algorithms is a forward error correction (FEC) algorithm. In the FEC algorithm, an FEC code, which is an error correction code to correct an error in a multiplexed frame to be transmitted to a receiving end, is added to the multiplexed frame, and the multiplexed frame with the added FEC code is transmitted to the receiving end. In the transmission apparatus on the receiving end, when the multiplexed frame with the FEC code added thereto is received, an error of the multiplexed frame is corrected using the FEC code added to the multiplexed frame. A description of related arts may be found, for example, in Japanese Laid-open Patent Publication No. 2003-188919.
However, in the related techniques, when a multiplexed frame with an added FEC code is transmitted, no consideration is given to suppression of an increase in transmission rate caused by implementation of the error correction capability.
That is, in the related techniques, an error of a multiplexed frame is corrected using only one FEC code added to the multiplexed frame. Therefore, it will be difficult to maintain a high error correction capability when a further increase occurs in transmission rate in the future.
In view of the above, a possible method of increasing the error correction capability may be to generate two error correction codes from a multiplexed frame and transmit the multiplexed frame together with the two FEC codes added thereto. However, in this method of transmitting a multiplexed frame together with two added FEC codes, the amount of information added to the multiplexed frame is twice greater, and thus the total transmission rate becomes higher than a standard value.